let foo = Symbol('foo')
let foo1 = Symbol('foo')

console.log(foo == foo1);
console.log(foo);
console.log(foo1);
console.log(Object(foo));

// 使用全局符号注册表
let fooGlobalSymbol = Symbol.for('foo')
let fooGlobalSymbol1 = Symbol.for('foo')
console.log(fooGlobalSymbol === fooGlobalSymbol1);
console.log(fooGlobalSymbol);

let undefinedSymbol = Symbol.for()
console.log(undefinedSymbol);

console.log(Symbol.keyFor(undefinedSymbol));
let undefinedSymbol1 = Symbol.for()

console.log(undefinedSymbol === undefinedSymbol1);

let a = 1
console.log(a++);
console.log(a);

let b = false
let c = b + 1
console.log(typeof c);
console.log({} && 1);